feat(runtime): Add a builtin actor for staking operations

[ekovalev]

The change includes:

• another builtin actor added to the Runtime to enable staking-related logic in programs (e.g. liquid staking etc.). The actor converts incoming messages into dispatchable calls and relays the latter to the Staking pallet;
• a set of types facilitating interaction with the staking builtin actor collected in a crate called gbuiltin-staking;
• a simple example of how contracts could interact with the builtin in question - for demo purpose only as real life scenarios would require more attention to details.

[breathx]

Blocked by #3924